This is a William Optics thing. My images look exactly the same, if not worse; I’ve been chasing this ghost for a while now.
If you figure this out, please let me know.
Yup, I have a William Optics ZS71 from 2013 which has exactly the same issue, I could write a book about the journey I have taken to try and sort it out.
I think that Tapio is 100% on the money with the issue being both tilt and probably back focus distance from the field-flattener to the camera. I would look at the tilt first as you won't be able to get the back focus distance 100% perfect until that is sorted out.
For me the tilt came from 2 areas, 1st from the focuser draw tube not being properly supported and 2nd from the interface between the focuser draw tube and the field flattener. I don't know about the ZS61, but the ZS71 does not have a thread focuser so the field flattener has to attach via the 2" compression ring. If this is the case for your scope, then I don't think you will ever get it perfect (but you can get it close). For me the biggest issue was consistency, some days I'd get it completely flat and others it would be terrible. It was very frustrating.
To improve the focuser I bought something described as "virgin skived PTFE self adhesive tape 10 x 0.25mm" which is what is already used in the focuser as a friction bearing. I got a 2m strip of it which is massive overkill, but gives you plenty to experiment and it's only a few £ a metre. I figured that to get a snug fit (snug, but not so tight it won't move) between the focuser body and the draw tube I used a piece of Tesa tape and a piece of the PTFE tape one on top of the other and added an additional 6 strips to the inside of the focuser :
This had the effect of making the focuser nice and rigid with no noticable slop or wobble. After this I added a 3rd thumbscrew to the 2" compression ring, for some reason it only ships with 2 thumbscrews at the top and a grub screw at the bottom:
I found that by tightening the bottom thumbscrew 1st and then the top 2 together afterwards I could get a fairly flat field. More importantly it is consistent and I know that I can get the scope out and follow my "tighten up" procedure and be fairly close to being square every time: